1. Field of the Disclosure
The disclosure relates generally to mesh reflectors for antennas and more particularly to mesh reflectors for antennas that may be used on spacecraft, and that may be stowed in a launch vehicle and subsequently deployed in outer space.
2. Background of Related Art
Over the past four decades, several styles of deployable mesh reflectors have been developed. The great majority of them were intended to approximate parabolic reflector surfaces, although any of them can theoretically be made to approximate other slowly varying surfaces, provided those surfaces do not have regions of negative curvature (i.e., are always curved towards the focus of the reflector). In more recent years, “shaped reflector” technology was developed and is gaining dominance in the space antenna field. So far, however, this technology has been limited to relatively small solid-surface (or segmented surface) reflectors due to limitations imposed by the fairing sizes of the launch vehicles on which they are flown.
A soft knitted mesh fabricated out of a thin metallic wire (preferably of gold-plated molybdenum) is commonly used to form the reflective surface of deployable radio-frequency (RF) antenna reflectors, especially for space-based applications (e.g., for communication satellites). The mesh may be placed and maintained in a desired shape by attaching it to a significantly stiffer net. One problem associated with the fabrication of such a mesh surface entails the ability to maintain the tension in the mesh within a certain desired range, and to terminate/cut the mesh edges in a manner that does not produce objectionable passive inter-modulation (PIM) or electro-static discharge (ESD), through the use of an appropriate mesh edge treatment.
The ASTRO-MESH Iso-Grid Faceted Mesh Reflector (hereinafter a “Type 1” reflector) is one example of a mesh reflector (see, e.g., U.S. Pat. No. 5,680,145). In this type of reflector, the mesh surface comprises a large number of triangular, substantially-flat facets. When viewed from a certain direction, the great majority of those triangles appear to be equilateral. The mesh facets are given their shape by being pulled behind a relatively stiff (ideally inextensible) set of highly tensioned straps forming a net with triangular openings. The net is pulled into shape by a set of springs pulling it backwards towards a similar (but possibly shallower) net disposed behind the mesh and curved in the opposite direction.
Another type of reflector is the Radial/Circumferential Faceted Mesh reflector (hereinafter a “Type 2” reflector). The most common examples of this type of reflector are the umbrella-style Radial-rib reflectors used on the TDRS antenna and on the folding-rib reflectors currently produced by Harris Corp, of Melbourne, Fla.
Yet another Type 2 reflector is shown and described in U.S. patent application Ser. No. 10/707,032, filed on Nov. 17, 2003, the entirety of which is hereby incorporated by reference herein. In this type of reflector, the mesh facets are generally of trapezoidal shapes bounded by a set of radial chords typically coincident with or near the location of, the reflector ribs, and by sets of chords forming concentric polygons extending between those ribs. Often, those substantially circumferential chords are made to more closely conform to the desired surface geometry by pulling down on them (i.e., in a direction pulling the surface away from the reflector focal point) with a set of adjustable tension ties. The loads in these tension ties are typically reacted by another set of chords forming a second set of concentric polygons disposed behind the set of polygons bounding the mesh facets.
Another type of reflector is known as a wrap-rib Parabolic-Cylindrically Faceted Mesh reflector (hereinafter a “Type 3” reflector). The wrap-rib reflector of Lockheed Martin of Bethesda, Md. has a mesh surface that comprises a relatively small number of facets each approximating a parabolic cylinder. Each of these facets is bounded by two curved parabolic ribs, an outer catenary member, and a part of the circumference of a central hub. The mesh used on these reflectors is designed to have very low shear stiffness and Poisson's ratio, which minimizes its tendency to “pillow” (or curve inwardly—i.e. towards the reflector focus—between the ribs). Typically, this type of reflector would only contain between one and several dozen facets.
With these current mesh reflectors, the surface of the mesh is divided into flat (or nearly flat) equally sized facets. The faceted reflectors are typically used to approximate the curved surface of an ideal parabolic reflector, which has a single main antenna lobe in their far field RF patterns. In application, however, these reflectors, having facets of equal size, stray from that ideal and produce relatively high side lobes in addition to the main lobe in the far field RF pattern. These side lobes, known as grating lobes, divert useful RF energy away from that main antenna lobe. These grating lobes are similar in shape to the main lobe and are spaced from the main lobe by an angle that often puts the grating lobes on areas outside the desired (and/or permitted) antenna coverage area, thereby causing undesirable interference with communications in those outlying areas.
There is a need for a technique for controlling the grating lobes produced by faceted mesh reflectors and spacing those grating lobes even farther apart from the main antenna lobe. There is also a need for a technique for diminishing the gain profile of these lobes to reduce interference with other communication signals.
The present disclosure is directed to overcoming one or more of the problems or disadvantages associated with the prior art.